This invention relates to relays for protecting a portion of a high voltage transmission line, and more particularly to characteristic timers for use with such protective relays.
Characteristic timers are frequently employed to determine the activation time of a protective relay. One such characteristic timer is described in my U.S. Pat. No. 4,438,476, assigned to the same assignee of the present application and which is incorporated by reference as if fully set forth herein.
U.S. Pat. No. 4,438,476 describes a characteristic timer having digital logic adapted to external circuitry for selecting one or more desired phase angles which determines the activation time of the timer and which is further adapted to external circuitry for selecting an integrating or non-integrating type function for the characteristic timer. Although the characteristic timer described in my above referenced U.S. patent functions well for its desired objectives, it is considered desirable that a characteristic timer be provided with further features for providing added margin against transient overreach for phase-to-ground faults of a reactance relay while providing shorter clearing times.
In the characteristic timer described in my above referenced patent, the characteristic remains non-integrating for a time delay. The integration begins only after the cut-off timer times out, and the magnitude of the operate level is selectable. The term "integrating" is meant to represent that the characteristic timer monitors an applied signal for more than one cycle of the applied signal and integrates the result. Conversely, "non-integrating" in meant to represent that the monitoring and the result of the characteristic timer are dependent on a half cycle of the applied signal.
To increase the security of the relay operation, the operate level is normally chosen to be equal to the highest expected transient level. As a result, the relay is precluded from operation on the occurrence of transients therefore substantially preventing transient overreach. However, because the operate level is selected in this manner, the speed at which the relay will operate, for faults within the protected zone will decrease because bonafide faults having levels less than the preselected operate level, will not cause operation of the relay until after integration begins at the conclusion of the cut off time period. On the other hand, reducing the operate level to increase speed may cause the relay to operate on transients rather than bonofide faults within the protected zone, thereby degrading the security of operation.
Accordingly, one object of the present invention is to provide a characteristic timer having features which result in improved relay performance.
Another object of the present invention is to provide a more secure scheme for reactance type measurement for ground fault protection.
A further object of the present invention is to provide added margin against transient overreach for phase-to-ground faults of a reactance relay while providing shorter clearing times.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of one form of the invention.